Process for applying fused metal coating onto metal base and adhesive used therein



United States Patent PROCESS FOR APPLYING F USED METAL COAT- lNG ONTO METAL BASE AND ADHESIVE USED THEREIN Norman W. Cole, Detroit, Mich.

No Drawing. Application February 7, 1952, Serial No. 270,525

26 Claims. (Cl. 117-22) m X This invention relates to a general method for fusion bonding a metal coating having desired special physical and/ or chemical properties in controlled uniform thickness onto any desired surface of a higher melting point base metal.

This application forms a continuation-in-part superseding two copending applications and consolidating the subject matter thereof with new matter disclosed for the first time in the present application.

The earlier of my copending applications, Serial No. 742,208 filed on April 17, 1947, and now abandoned, disclosed the general coating process of the present application including steps of applying a thin liquid film of adhesive to the surface of the base metal to be coated, the application of an excess of granulated coating metal over such liquid film, removing all but the single layer of granulated coating metal retained by such liquid film,

, and heating by suitable medium to a temperature below the melting point of the base metal at which the granulated coating metal will fuse, displace the adhesive and'bond to the base metal.

Whereas the preferred method of heating disclosed in such earlier copending application comprised a controlled nonoxidizing furnace atmosphere, the second of my copending applications, Serial No. 83,288, filed on March 24, 1949, and now abandoned, was directed primarily to an improvement in the method of heating in the fusing step of such process consisting in the use of a liquid bath and more particularly the use of a fu ed boric oxide heat- 1i n gfiftxh which produced remarkably improve results rom t e standpoint of smoothness of coating, perfection of uniform fusion bond, complete protection of coating particles and base metal from oxidation, and freedom from distortion.

The present application adds the disclosure of a particular method for applying the particles of coating metal over the film of liquid adhesive which makes it possible to use a substantially finer particle size than that retained by a 325 mesh screen, which had been taught by my earlier copending application to define the lower limit of particle size which could be satisfactorily applied in a uniform layer. This new method of application is characterized by a gradual sprinkling of coating metal particles, as through a screen or sieve, over the film of liquid adhesive in contrast to the earlier disclosure of a sudden pouring of an excess of coating metal particles onto such surface.

The coating process and materials used therein forming the subject matter of this consolidated application are particularly directed to provide a method for producing a fusion bond" between coating and base metals as characterized by a solid solution or alloying of the base and coating metals at the contact surface; to provide a method for binding particles of refractory or relatively high melting point materials such as tungsten carbide, chromium boride or diamond particles to the surface of a base metal in a coating matrix capable of fusion bonding at a temperature below the melting point of such base metal; to provide for surface coating of entire objects or of any desired limited surface areas thereof whether sgch surfaces be horizontal, vertical, lower, irregular, remote, internal or external, or whether they be broad in area or limited to a thin edge; and they are particularly directed to provide an exceptionally uniform coating with extremely accurate control of coating thickness anywhere within the range of .001 to .025 of an inch.

The basic steps of the process as applied to plain coatings comprise the application of a thin film of liquid adhesive (preferably water soluble proportions of water glass and borax into which a fine metal powder is mixed with sufficient water to provide a paint consistency) to any surface of the metal article to be coated, the application to the thin film of adhesive of a substantially single layer of granulated noneutectic alloy coating metal having a particle size exceeding the thickness of the adhesive film, and the heating of the coated surface to a temperature below the softening point of the base metal whereat the coating metal becomes plastic, displaces the nonmetallic ingredients of the adhesive and fusion bonds with the base metal.

Where it is desired to bond refractory particles in the surface coating, the basic steps include the application of the adhesive film followed by the application of a mixture of the refractory particles with the granulated binding metal and the heating of the coated surface to the bonding temperature; or where a denser coating of refractory particles is desired or where differences in the size and/or specific gravity of the refractory particles and the binder metal make it difficult to obtain a uniform mixture, the process may be varied by applying to the ahesive film a layer of refractory particles alone followed by the drying of the adhesive, the application of a second coating of adhesive, the application of the granulated binding metal and the heating of the coated surface to the bonding temperature. Depending on the materials used, the latter procedures may result in a fusion bond between the base and binder metals alone with the refractory particles merely embedded in a matrix of the binder metal or the binder metal may alloy or fusion bond with the refractory particles as well as the base metal; and depending on the relative size and quantity I metal as a result of the relatively greater affinity between the base and coating metals.

After the film of liquid adhesive has been applied to the base metal by any commercial painting technique, the preferred method of applying the particles of coating metal onto the film of liquid adhesive, particularly where a particle size fine enough to pass through a 325 mesh screen is employed, consists in sprinkling the coating metal particles gradually over the surface to be coated as by screening it through a sieve. The preferred method of heating, where the fusion bonding temperature is over 1500 F., consists in submerging the article, with the coating metal applied and the adhesive dried, into a fused liquid bath of boric oxide having a controlled temperature corresponding to the fusion bonding temperature for the particular coating metal employed.

The novel scope of my invention is believed to comprehend the basic process as used in the two general classes of application indicated above, namely plain coating and refractory particle coatings, the particular preferred adhesive used in such process, the preferred method of applying particles as used in such process, and the preferred method of heating as used in such process.

I am aware of numerous patented and unpatented processes for coating base metals which involve the use of powdered coating metals and nonmetallic vehicles for applying them. Among these are a number of processes in which nonvolatile constituents of the vehicle form a permanent part of the coating and in which no fusion bond takes place between the coating and base metals.

A number of other prior processes are known wherein the vehicle does not form a permanent part of the coating but wherein it is premixed into a paste with powdered coating metal. Thus, a process has been developed for fusion bonding tin to a base metal such as black iron wherein powdered tin is mixed in suspension in a lacquer or resin binder and thereupon applied to the iron base metal. After oven drying to drive out the lacquer solvents, the surface is heated to the fusion temperature leaving a film of carbon which may be scrubbed ofl.

2,694,647 Patented Nov. 16, 1954 In another early process, powdered copper is mixed in a heavy paste of sodium fluoride, sodium silicate and borax and applied to fiat iron plates as by a printing roller, the paste vehicle serving to completely envelop and protect the powdered copper from oxidation as it settles down against the iron plates during heating; or as a variation the powdered copper may be applied on top of the other paste constituents.

Still another process involves the mixture of hard refractory particles; such as tungsten carbide with particles of a relatively low melting point bonding metal in a sodium silicate paste which may be applied to earth engaging tool surfaces subject to abrasive wear and heated in an atmosphere controlled furnace.

However, in none of these processes is av thin film of liquid adhesive first applied which is suitable for retaining only a single layer of subsequently applied granulated coating metal, nor is any method disclosed for applying a uniform substantially single layer of granulated coating metal such as to provide for accurate control and uniformity of thickness. In the first class of processes mentioned above wherein the vehicle forms a permanent part of the coating, the physical characteristics of the surface are necessarily limited to those of the vehicle. The second class of processes wherein the powdered coating metal is first mixed with the vehicle is either limited to the production of very thin coatings in the order of .001 inch because of the limited quantity of powder which may be mixed into any vehicle while retaining a consistency which will permit it to be uniformly applied, or else involves a heavy thick paste which is not susceptible to uniform application or accurate control of thickness by any production method. In contrast, the present process is adapted to produce accurately and uniformly any desired thickness of coating within extremely wide limits (.001" to .025") through a proper selection of the size of granules which are applied in a uniform single layer to the adhesive. Furthermore, either the suspension of powdered metal in a vehicle or the application of a powdered metal to a relatively thick paste such as would provide complete envelopment of the metal powder as it settled through the paste and onto the base metal, is believed to result in a fundamentally dilferent type of bonding action than is obtained in the present process as will be hereinafter explained in detail.

While the process disclosed in my earlier copending application Serial No. 742,208 was not specifically limited to any particular method of heating to the fusion bonding temperature, torch or furnace heating, the latter either in a plain or controlled atmosphere, were the only methods which had been employed at the time of filing or suggested in such application. Although coatings of commercial quality for soome purposes can be obtained in a special reducing atmosphere such as hydrogen or by careful application of a suitable protective flux such as boric oxide where an oxidizing atmosphere is encountered, there are numerous practical difiiculties in obtaining uniformly smooth and imperforate coatings with such heating methods.

One imperfection which was frequently encountered is the formation of small depressions or poc marks in the coated surface which have the appearance of being formed by small trapped gas bubbles creating localized pressure points on the plastic surface of the coating metal while it is being fused. It has been found that careful control of the protective flux to preserve it in an anhydrous condition helps to minimize the appearance of these depressions. However, my numerous attempts to meet this problem in an economical and commercially acceptable manner were of limited success in applications requiring a uniform high quality surface until I employed the liquid fused boric oxide bath which served both as a heating medium in raising the temperature of the coated products to the fusion bonding point and to completely protect such parts from oxidation during the heating operation.

As mentioned above, the process as disclosed in my prior copending applications was subject to a limitation in the minimum size of coating metal which could be used, a 325 mesh screen size being taught as marking the lower limit of particle size. It has been my experience that if finely divided coating metal of a particle size which would pass through a 325 mesh screen is poured onto the adhesive film on the surface of a base metal and the excess is then removed by gravity and jarring, the coating metal tends to adhere in irregular lumps rather than in a uniform or single layer as required by the process.

While I do not completely understand the cause for such lumping of finely divided coating metal it may be that the capillary action obtaining in the case of finely divided particles is such as to cause the liquid adhesive to rapidly soak up into the excess of material applied and thereby cause hills or piles of the coating particles to adhere together whereas in the case of granulated particles of a size exceeding that which will pass through a 325 mesh screen, the individual particle size is of a thickness sufficiently greater than the thickness of the film of adhesive such as to inhibit such capillary action beyond the substantially single layer of particles which directly contact the adhesive film. In any event, all attempts to extend the range of particle size in the direction of finer particles, such as would pass through a 325 mesh screen, by thinning the consistency of the adhesive or by attempting to spread it out into a thinner film were unsuccessful.

On the other hand, while very finely powdered metal (under 20 microns) could be mixed directly into the adhesive and spread onto the base metal in a uniform paintlike film either by brush or spray gun, such method has been found to be impracticable with a particle size exceeding 20 microns since the resulting mixture departs from the required paintlike consistency with solid particles that are not adaptable to uniform spreading with paint brush, spray gun, or other painting techniques. Thus, until recently there was a range of coating particle size (20 to 44 microns, the latter corresponding to a 325 mesh screen size) for which no suitable method of application was known consistent with desired accurately controlled uniform thickness coatings and with feasible production methods of application.

Coating metal particles in the size range of 20 to 44 microns may be successfully applied to the film of liquid adhesive on the surface of a base metal if screened on through a mesh sieve to an excess which is then removed by rapping or jarring the base metal article. The effect of so screening the particles on appears to be such as to use up the absorption or capillary spreading properties of the film of liquid adhesive with the first layer of coating particles to fall over the surface of the film as they gradually descend onto such surface from the sieve. In any event the excess particles applied by such method fail to adhere in lumps or irregular thicknesses, as when such particles are poured on suddenly in excess quantity, but instead all but a smooth uniform layer are readily removed by gravity when the part is rapped or otherwise jarred to impart vibration thereto.

Thus the present disclosure of what may be termed a screen on method of applying the coating metal extends the range of particle size to which my process is applicable substantially below the former 325 mesh limitation and to a particle size less than half of that formly thought to be the minimum, closing the gap between the very small particle size (under 20 microns) which may be uniformly applied directly in the adhesive and the size of 325 mesh and over disclosed by the process of my prior copending applications, and making it possible to apply uniformly and accurately controlled thicknesses of coating metal of any size ranging from the finest bag powder up to 30 mesh representing the complete output of conventional mills for crushing metal into granulated and powdered particles.

Accordingly, it is the principal object of the present invention to provide a new process for surface coating any base metal with a uniform and accurately controlled thickness of one or more metals, as in providing special physical characteristics not possessed by the base metal such as wear resistance, hardness, high tensile strength, special appearance, resistance to low or high temperature oxidation, corrosion, abrasion or acid.

Another object is to provide a process for binding a uniform layer of refractory particles onto the surface of a base metal.

Another object is to provide for uniform coating thickness through a single layer application of granulated coating particles.

Another object is to provide for accurately controlled thickness of coating through single layer application of granulated coating particles which are preselected and graded 1n size.

Another object is to provide for exceptional range of coating thickness through the use of noneutectic alloy coating metal having a substantial plastic temperature range between liquidus and solidus points.

Another object is to extend the range of heavy coating thickness through the use of a metal powder in the adhesive having solid properties at the fusion bonding temperature to inhibit slipping of the plastic coating metal on inclined surfaces.

Another object is to provide an adhesive for use in this process which may be distributed on the surface to be coated in a uniform thin liquid film by conventional painting techniques.

Another object is to employ an adhesive which will include a minimum of nonvolatile, nonmetallic residue to be displaced and gotten rid of in the fusion bonding and succeeding steps.

Another object is to employ an adhesive having adequate holding power to firmly retain relatively large granules of coating metal throughout all steps of the procelss leading to the fusion bonding of such coating meta Another object is to employ in such adhesive an inhibitor to prevent running of the containing metal on inclined surfaces when it reaches a fused state.

Another object is to provide a heating medium for use in a process of this type which will also serve to completely protect the part heated from oxidation as well as from the formation of irregularities in the surface encountered with furnace and torch methods of heating.

Another object is to provide a liquid bath for this purpose which will provide protection against oxidation both during the period of heating and during the period 3f lclooling after the article is withdrawn from the liquid Another object is to provide a liquid bath which is inert with regard to any chemical action with the parts submerged therein, nonvolatile at the temperatures employed, which may be readily removed from the part after it has served its purpose, which is both stable and nontoxic at the fusion bonding temperatures, and which is economical and can be recovered conveniently for reuse.

Another object is to provide a heating medium permitting a more rapid heating than is possible with a gaseous heating medium, which will facilitate the uniform heating of all surfaces of any parts introduced therein, which will adhere to parts removed therefrom in sufii cient quantity to promote even cooling and to minimize distortion as well as to protect the part from oxidation during cooling, and which may be readily removed from the parts submerged therein and leave the surfaces clean and bright.

Another object is to provide a general surface coating process applicable to a wide range base metals, coating metals and commercial requirements.

Another object is to provide a surface coating process adaptable to automatic methods for quantity production.

Another object is to provide a surface coating requiring no special skill on the part of the operator and Ivlgich is economical with respect to time, materials and a or.

Another object is to provide for the use of any and all particle sizes of coating metal extending from the finest powder up to as large as 30 mesh by a method which will accommodate the application of the uniform layer of such coating particles and the accurate control of thickness over entire surface areas of any extent, contour, and inclination.

These and other objects will be apparent from the following more detailed description of the process, the materials used therein and specific examples of proven applications.

Plain coatings There are certain inherent requirements which to the best of my knowledge form the only limitations for defining the scope of the process in its application to plain coatings.

1. The base metal must be capable of retaining its shape at the bonding temperature and must have suitable fusion bonding properties with the coating metal at a temperature below the melting or softening point of the base metal.

2. The coating metal is preferably a noneutectic alloy having a substantial plastic range between liquidus and solidus points below the melting or softening temperature of the base metal, such plastic range being essential where coatings of substantial thickness (in excess of one or two thousandths) are to be made on inclined surfaces since pure metals or eutectic alloys with abrupt melting points producing a sudden transition from solid to liquid state will invariably either run off or fail to properly bond under such conditions. a

3. The adhesive must have a liquid paintlike consistency capable of thoroughly wetting and covering the surface of the base metal in a thin film when applied thereto by conventional painting techniques such as brushing or spraying so that the resulting film has a smaller thickness than that of the particles of coating metal applied thereto; must be capable of retaining substantially a single layer of granulated particles of coating metal when applied thereto in excess; must be capable of holding such layer of granulated coating metal both during the preliminary handling of the article and during heating to the bonding temperature; must provide necessary fiuxing action to promote bonding and its displacement from contact with the base metal by the coating metal when the bonding temperature is reached.

The base metal may be prepared for the process by merely cleaning the surface of any grease or dirt such as would interfere with proper bonding or which might form inclusions tending to weaken the bond. It is not necessary that the base metal be shot blasted to roughen the surface as is the case with certain flame spraying processes but it is rather preferable that the surface be smooth where it is desired to have a smooth finished article. This fact permits the use of hard base metals not adaptable to conventional spray welding processes.

The adhesive which I have found most satisfactory for a wide range of applications consists of equal parts by volume of general purpose commercial Marie q1va tl e;, gl a approximately 40 Baum; appro. imate a ai-silica ratio 15311} with a saturated solution of com m'ercial sodiiim'borate (borax approximately two grams per cc. of water) in solution with three parts by volume of water. The use of sodium silicate by itself is not adequate regardless of the extent of dilution in water since it lacks desired wetting properties making it difiicult to apply in a uniform thin film and since when applied in such thin film it does not have sufiicient elastic properties to permit it to expand with the base metal upon heating to the fusion bonding temperature and as a consequence tends to crack, spall and flake off during heating with resultant irregular and undependable coatings. The recommended water soluble proportion of borax in the adhesive imparts flexibility thereto which overcomes this cracking during heating, as well as to add fluxing properties tending to promote a uniform and excellent fusion bond between base and coating metals without unduly weakening the adhesive holding properties for which the sodium silicate is relied upon.

While I have attempted to use the above adhesive, which may be termed plain adhesive, on applications limited to horizontal or nearly horizontal surfaces, it is not entirely satisfactory even for such limited purposes due to the fact that it is waterlike in appearance and has to a limited extent the same tendency as water to gather on the surface of metal in pools rather than thoroughly wet the surface in a thin uniform unbroken film, and its colorless transparency makes it ditficult to tell from appearance what areas are thoroughly and properly wetted. Accordingly, such plain adhesive by itself has been found in practice to be difficult to control with dependable results and the addition of fine metal powder (in the order of 20 microns and under) mixed into the adhesive in sufiicient quantity to give it a dark appearance and a paintlike consistency is greatly preferred and recommended for all applications. This metal powder serves a second important function where it is desired to coat inclined surfaces in appreciable thicknesses where even a noneutectic plastic coating metal may have a tendency to slip at the fusion bonding temperature. By using a metal powder in the adhesive which has a higher melting point than the fusion bonding temperature, such powder has the effect of providing anchoring points against the slippage or flow of the main coating metal when it reaches a plastic state. In this connection it has been found that the same alloy as employed in the main granular coating may be employed powdered form in the adhesive for the purposes mentioned since the melting point of the same material in powdered form appears to be appreciably higher than in the larger granulated form and at least to the extent required to prevent slippage serves substantially as well as the powder of a different metal having a much higher melting point. The powdered metal in the adhesive may be chosen to serve certain other purposes such as to lower the temperature at which a fusion bond will take place, to promote the alloying of two or more metals used in the coating, to add to the hardness or abrasive resistance of the coating metal proper, etc.

Experiments varying the proportion of silicate and borax from those set forth above in the formula for my preferred adhesive have demonstrated that superior results can be obtained only when soluble proportions of these ingredients are used and that the adhesive is rendered inadequate in holding power if the one part of saturated solution of borax is increased to more than four parts.

Since the adhesive in my process is relied upon to hold relatively large granular particles greatly exceeding in thickness that of the applied film of adhesive, not only during preliminary handling and against the wash of submersion in a viscous liquid bath, but also to continue to hold such particles throughout the heating to the fusion bonding temperature, my adhesive is to be distinguished from any volatile or organic vehicle, any holding properties of which would necessarily disappear during heating to temperatures of 1750 to 2300 F. within which range my process is most frequently used.

It has been found desirable to use as dilute a solution of adhesive as is possible consistent with the proper holding of the coating material since the smaller the quantity of nonvolatile materials in the adhesive the more easily and quickly such materials will be floated to the surface when the bonding between coating and base metals takes place. While variations in the proportions of sodium silicate and sodium borate may be made with satisfactory results within the range indicated above, no change from the equal proportions of my preferred adhesive has been found necessary in any application involving bonding temperatures ranging from 1750 to 2300 F. If more than four parts of borax solution for one of sodium silicate are used a heavy precipitate results which destroys the paintlike consistency of the adhesive, interferes with its uniform application in a thin film as required and unduly weakens the holding properties of the decreased proportion of adhesive sodium silicate. If as much as eight parts of borax solution are used the entire mixture will in less than one minute turn into a solid mass of jelly totally unfit for application as an adhesive film.

In general any adhesive used in the process should meet the following requirements:

1. It should be readily applicable in a uniform thin liquid film by hand or automatic painting techniques, as distinguished from a paste or jelly which does not lend itself to thin or uniform application, especially on broad and irregular surfaces;

2. It should have the quality of thoroughly and evenly wetting the base metal;

3. It should be capable of collecting and holding a single layer of granulated particles of the coating metal when the same are poured or sprinkled onto the adhesive and to continue holding such particles during ordinary handling at room temperature as well as against the washing action of submersion in a viscous liquid heating bath and throughout the heating of the surface to the bonding temperature;

4. It should be capable of so holding the coating metal on irregular, vertical and lower surfaces in order to have general application to articles of any form or contour;

5. It should be capable of expanding with the base metal during the heating operation without spalling, blistering or flaking off before the bonding temperature is reached;

6. It should provide a fluxing action in promoting the fusion bond between the base metal and coating metal;

7. It should have a viscosity and surface tension low enough at operating temperatures to permit it to free itself from the base metal and allow the coating metal to make a fusion bond with the base metal;

8. It should comprise materials having inert chemical properties to avoid compounding with either the base metal or coating metal and any nonvolatile residue should be readily removed from the coating surface after the fusion bond has taken place and the surface is cooled to handling temperature;

9. It should have volatile properties which are nontoxic and inoffensive; and

10. It is of course also desirable that the adhesive should be inexpensive, readily prepared from available materials without special skill and free of deteriorating qualities.

The preferred adhesive described above fully meets each of these requirements in every application tested to date and forms a highly important factor in the uniformly successful results obtained, although it is recognized that having demonstrated the successful use of an adhesive of this type in the present process and having explained the specific requirements of such adhesive, a search for substitute ingredients might well reveal other iatisfactory materials which could be used in the adesive.

The coating metal is prepared in a granulated form, preferably somewhat coarser than will pass through a 325 mesh screen when applied in excess by pouring over the film of liquid adhesive or by any other sudden means. However, as mentioned above, if particles of coating metal are sprinkled on gradually, as through a mesh sieve, a through 325 mesh particle size may be employed including particles within the range of 20 to 44 microns.

The coating particles should be irregular in shape, particularly where the larger sizes are employed, in order to avoid any tendency to roll during their application to the adhesive. When applied in the form of round shot such tendency to roll has been found to result in incomplete and nonuniform coverage. Furthermore, the production of granulated material in shot form which is ordinarily accomplished at high temperatures usually results in a thin oxide coating around each particle which interferes with proper fusion bonding. If initial steps of refinement of the coating metal involve a shot process, the shot should accordingly be crushed to break up any oxide coating as well as to provide irregular particles. With particles coarser than 325 mesh, a single layer is readily applied and the size of such granules will accordingly determine the ultimate thickness of the coating. Final smooth coating thicknesses ranging from one thousandth to twenty-five thousandths of an inch have been successfully obtained in the various applications made to date.

It is to be noted that the use of relatively coarse granules rather than fine powder results not only in a uniform single layer but in each granule projecting above the surface of the adhesive since the adhesive forms a relatively thin film over the surface of the base metal. The surface tension of the adhesive in wetting each granule produces a surface tension meniscus having a resultant force tending to pull such granules into contact with the base metal. Since the greater afiinity of the base and coating metals is depended upon to cause the coating metal to displace the adhesive at bonding temperatures and since such affinity necessarily depends upon a very close association if not actual contact between the base and coating metals, it is believed that the surface tension forces mentioned have an important function in producing the complete and excellent fusion bond characteristic of all applications.

Furthermore, the relatively large exposed surface area of metals in a powdered state greatly augments the oxidizing property of such metal and the difiiculties of securing a thorough and perfect fusion bond. It is well known that most metals when exposed to the atmosphere even at normal temperatures acquire an oxide film at the surface. While such film may be imperceptible to the eye its existence even in the slightest degree is recognized as an obstacle to securing uniform and perfect fusion bonds in any kind of welding process. as in the case of aluminum which acquires an oxide film almost instantaneously upon exposure to atmosphere. Where a metal coating is prepared in a finely divided or powdered state, any oxide which forms at the surface of the powdered grains necessarily comprises a much greater proportion of the total mass of coating metal than where the same metal is prepared in the form of relatively coarse granules. For example, the exposed surface area of particles of one thousandth of an inch diameter is ten times as great as the exposed surface of the same quantity of material in the form of particles ten thousandths of an inch in diameter. By using single layer granules of the largest size consistent with the ultimate coating thickness requirements, a minimum surface area is exposed and the tendency and effect of any oxidation is accordingly likewise minimized.

In this connection the use of a liquid heating bath, preferably of fused boric oxide, has been found to be of the utmost importance. By using such liquid bath complete protection against oxidation during heating is provided for the exposed surfaces of the coating particles projecting above the surface of the adhesive film which promotes the uniformity and excellence of fusion bond characteristic of the process, as well as to keep the entire base metal and final coating surface completely free of oxidation during and after heating. This has made possible, for example, the coating of heat resisting metals such as used for jet engine parts which even in a dry hydrogen atmosphere acquire a deep discoloration and tarnishing, while such metals are left completely bright and clean in every respect when heated in fused boric oxide. It has also made possible the hard facing of air hardening steels which have been found to crack under other methods of welding.

The preparation of a liquid heating bath employing fused boric oxide is accomplished merely by heating bloric acid (H3803) in a heating pot to a temperature 00 F. until all boiling ceases. The resultant boric oxide (B203) forms a clear liquid of high viscosity, completely inert and nonvolatile within the temperature range of 1500 to 3000 F.

Coated articles may be directly plunged into the liquid bath of boric oxide after such bath has been brought to a temperature corresponding to the proper bonding temperature for the particular coating involved. If the heat transfer to the articles substantially reduces the temperature of the bath as a whole, they must necessarily be left in the bath for a sulficient time to permit the temperature of the bath to come up to the bonding temperature. If, on the other hand, the bath is of a sufficient volume in comparison with the coated articles submerged so that the temperature of the bath is not appreciably lowered it is only necessary to leave the part in the bath for a sufficient time to permit its temperature to rise to the fusion bonding point. The time required for such temperature rise has been found to be substantially less in the case of the present liquid bath than in the case where furnace heating is employed.

On the other hand, While the dilficulty of preventing oxidation made it deisrable in the case of furnace heating to remove the part as soon as possible, the protection from oxidation in the case of the present liquid bath is so complete that the parts may be left in the bath at the fusion bonding temperature for an indefinite period of time without ill effects. In fact, it has been found desirable in applications where extremely smooth and non-porous coatin s are required to permit the part to remain in the liquid bath at the bonding temperature for a protracted period to permit surface tensions operating to smooth out the coating metal in its plastic fusion bonding state to perform their maximum smoothing function.

Accordingly, the only vital requirements in achieving successful results with this liquid bath are that the temperature of the part at some time after it is introduced into the bath be raised to the fusion bonding point and that where other than horizontal surfaces are being coated, the temperature should not exceed that at which the fusion bonding takes place by a sufficient amount to cause slippage or running oif. While the plastic range of the particular coating metal will of course determine the plus or minus temperature tolerances I have found that in most applications, a plus or minus ten degrees is consistent with satisfactory results.

In order to achieve rapid heating the bath may be agitated either through the movement of the part to be coated or through some other mechanical agitating means. However, in all commercial applications for which the bath is being used at present, no agitation of the bath has been found necessary.

The viscosity of the boric oxide at l50G to 1800 F. is like that of a heavy syrup While at a temperature of about 2000 it achieves a viscosity of a thin syrup. Thus, when any part is removed from the bath after the fusion bonding temperature has been reached a substantial quantity of the liquid boric oxide adheres to the surface of the part offering complete protection against oxidation while the part is cooling. which may be either rapid or slow depending upon the heat treatment of the part desired.

When the part has cooled to the room temperature, the

boric oxide will have completely hardened and may be readily flaked off any smooth surface. Substantial quantities may be jarred off of even rough surfaces. Thus a major portion of the boric oxide adhering to the part may be recovered in an anhydrous state and put back into the bath for further use. Any quantity of the boric oxide which continues to adhere to the surface of the part may be readily dissolved in warm or boiling water. Since the solubility of boric oxide is very high in boiling water and very low in cool water a further recovery of the boric acid may be made by merely cooling the water whereupon recrystallization of the boric acid takes place. This may in turn be calcined and again used in the bath.

It will be noted that no fumes are given off by the boric oxide bath as frequently encountered in conventional salt baths and that the bath does not in any way deteriorate upon continuous use. For example, one bath has been used in commercial operation for two and one-half years without servicing in any way except by replenishing the boric oxide dragged out through removal of the parts heated therein.

This method of heating readily lends itself to continuous conveyor operations wherein parts are automatically lowered into one end of a liquid bath tank and raised out of the other end.

While the liquid heating bath of boric oxide is much preferred over any other as a medium of heating and constitutes a highly important step in improving the results of my process as well as in broadening the commercial possibilities thereof to numerous fields which otherwise would be excluded, it is possible where extreme smoothness of surface and complete protection from oxidation are not critical requirements to obtain commercial results with other heating mediums. Thus it is possible to perform the heating operation in an atmosphere controlled furnace in which case the article may be. placed in the furnace as soon as the adhesive has been properly dried. If, on the other hand, the heating is accomplished by torch or a furnace having an oxidizing atmosphere, it is desirable to apply protective flux prior to heating to the bonding temperature. A satisfactory flux for this purpose is the same boric oxide employed in the bath which is preferably applied in anhydrous powdered form after the adhesive has been warmed in drying so as to permit the flux to melt and thoroughly cover the coated surface. (It is to be noted that this method divorces the holding function of the adhesive applied before the granulated coating metal, from the protective function of the flux, applied after the coating metal, as distinguished from the process for applying copper powder mentioned at the beginning of this specification wherein a protective paste having no adhesive function is either employed as a vehicle for applying the copper powder or else is applied before the powder so as to depend upon the powder settling through the protective paste under the influence of gravity to achieve envelop ment and protection from oxidation forming the principal object of such process.)

Heating to the fusion bonding temperature may be as rapid as desired and the temperature held no longer than necessary to permit the particles of coating metal to displace the adhesive and bond with the base metal. For most operations one to two minutes at the fusion bonding temperature is adequate to secure the completion of such bond. The article may then be removed and air cooled or quenched or may be oven cooled depending on the particular heat treating requirements of each case.

The nonvolatile residue of the adhesive which will have been floated to the surface of the coating metal may be removed by Washing or scrubbing in warm water.

Refractory particle coatings Where it is desired to bond refractory particles such as tungsten carbide, chromium boride, diamonds or other particles having a high melting point to the surface of a cover over the hard particles with the binder metal. The size of the granulated particles of the binding metal will determine the extent to which the refractory particles are embedded and may be chosen to permit the refractory particles to project beyond the binding metal to any extent as might be desired in the manufacture of abrasive tools.

The refractory particles employed in a surface coating should be of a size exceeding 325 mesh, or else applied gradually as in the case of plain coating metal particles in order to secure a uniform smooth layer distribution. Where relatively large refractory particles are used it may be desirable to increase the proportion of sodium silicate in the adhesive as well as to provide a somewhat stronger solution by decreasing the water content in order to develop stronger adhesive qualities than required for normal coatings.

It is also possible where particularly smooth coatings are required, or where a substantial thickness of relatively fine refractory particles is desired, to build up multiple layers of relatively fine particles by repeating the successive application of adhesive, refractory and binding metal particles, the refractory and binding metal particles being either mixed or applied in separate layers, and the adhesive being applied in each case between successive applications of individual layers, all layers being fused in a single heating. In following this multiple layer procedure it is usually necessary or desirable to provide binder metal for each layer of refractory particles rather than to apply successive layers of refractory particles followed by a single layer of binder metal in which latter case the penetration and distribution of binder metal may prove inadequate to satisfactorily bind all refractory particles. In following this procedure it is also frequently desirable to thin the adhesive applied to the outside of particle layers by adding plain adhesive to that in which the metal powder has been mixed providing a consistency of thin paint since the consistency desired for initial application directly to the surface of the base metal may be irregularly soaked up into the pores of the under layer causing it to dry and pile up before it can be properly smoothed out, as where applied by brush, the thinner consistency providing a better uniform film over the irregular surface of a prior layer of coating particles.

Specfic applications The above described process has been successfully used in actual commercial production to coat the following representative base metals:

In such applications the following coating or binder metals have been used:

Various chromium-boron-nickel alloys (particularly well adapted to the process because of characteristic long plastic ranges, suitable bonding temperatures and desirable physical properties);

Bronze of the 90% copper, tin type; and

Brass of the 70% copper, 30% zinc type.

In a large number of the above applications involving extreme wear resistance requirements refractory particles of either tungsten carbide or chromium boride have been incorporated in the coatings.

In all of the above applications a fine metal powder has been employed in the adhesive, the following being representative:

Tungsten carbide and chromium boride of the same types as used in the refractory coatings except in a finely powdered form under microns;

High melting point chromium-boron alloys with small amounts of iron or nickel;

Chromium-boron-nickel alloys of the same type as employed in the coating metals;

Pure nickel; and

Pure iron.

In addition to the above representative commercial applications the process has been successfully demonstrated experimentally to have numerous other applications. For example, copper has been coated with a relatively lower melting point hard alloy of chromium, boron and nickel; steel has been coated with very high percentage nickel alloys; and diamonds and boron carbides have been employed in refractory coatings.

Such applications have involved fusion bonding tem peratures largely within the range of 1750 to 2300 F., depending in each case upon the plastic temperature of the particular coating or binding metal used.

It will be clear to anyone skilled in the art that innumerable other applications can be made by following the general requirements set forth above which may be summarized as follows:

1. The base metal should be capable of bonding with the coating metal at a temperature below the softening point of the base metal;

2. The adhesive should be as thin as possible consistent with proper holding qualities and may be adapted to hold larger, heavier granules on inclined or lower surfaces by using a more concentrated solution and/ or by increasing the proportion of silicate used, may be adapted to bonding temperatures under l750 by increasing the proportion of borax, should include a high melting point finely divided metal powder aggregate preferably having a particle size under 20 microns mixed into the adhesive to a paint consistency which will permit the same to be applied to the surface of the base metal in a uniform, thin, liquid film by conventional painting techniques, which powdered metal aggregate may be the same material as the coating metal itself or as the larger granulated refractory particles to be bound into the surface of the coating metal or some other metal chosen to provide special alloying, hardening, or other properties in additin ot the main functions of providing a paint consistency and wetting property and preventing slippage of fused coating metal;

3. The coating or binding metal should be granulated in a size larger than will pass a 325 mesh screen and such as will result in a desired final coating thickness, or if through 325 mesh particles are employed in the range of 20 to 44 microns, they should be gradually applied as through a mesh sieve; should be a noneutectic alloy having a substantial plastic range; should have fusion bonding properties at a temperature below the melting point of the base metal; and may be selected for any desired physical properties; and

4. Heating should preferably take place in a fused boric oxide heating bath, having a temperature corresponding to the fusion bonding temperature of the particular coating or binding metal used, although somewhat less perfect but commercial results may be obtained for certain applications in an atmosphere controlled furnace or in the presence of a protective flux where a plain atmosphere or heating torch is employed in fusing and the coating.

While the preferred process and adhesive used therein have been described above in detail as applied to both plain and refractory types of coatings, it is understood that numerous modifications in details might be resorted to without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. A process for fusion bonding a metal coating onto a metal article having a relatively higher melting point which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of water soluble proportions of water glass and borax into which a metal powder having a maximum particle size in the order of 20 microns and a melting point above the fusion bonding temperature has been mixed in sufficient quantity and with sufiicient water to provide a paint consistency, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating in granulated particles exceeding the thickness of said adhesive film and within a range of through 30 retained on.325 mesh having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F., removing from said article the excess of granulated coating alloy other than the substantially single layer retained by said adhesive film, drying said adhesive, and heating said surface to a temperature within said plastic range at which said coating alloy will displace the nonvolatile water glass and borax ingredients and fusion bond with said metal article by submerging said article in a fused bath of boric oxide having a corresponding temperature.

2. A process for fusion bonding a metal coating onto a metal article having a relatively higher melting point which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of water soluble proportions of water glass and borax into which a metal powder having a maximum particle size in the order of 20 microns and a melting point above the fusion bonding temperature has been mixed in sufficient quantity and with sufficient water to provide a paint consistency, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating in particles within the range of 20 to 44 microns having a substantial plastic temperature range between the liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F. by gradually sprinkling the coating alloy over the liquid adhesive film by screening it on through a sieve, removing from said article the excess of coating metal particles other than the substantially uniform layer retained by said adhesive film, drying said adhesive, and heating said surface to a temperature within said plastic range at which said coating alloy will displace the nonvolatile water glass and borax ingredients and fusion bond with said metal article by submerging said article in a fused bath of boric oxide having a corresponding temperature.

3. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a volatile liqpjgl vgggcle, a nonvolatile nonmetallic inorgamc'M ith plastic holding properties at the fusion bonding temperature, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 15 to 2500 F., removing from said article the excess of granulated coating alloy other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said coating alloy will displace said nonmetallic adhesive material and fusion bond with said metal article.

4. A process for fusion bonding a coatingonto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a volatile liquid vehicle, and a nonvolatile nonmetallic inorganic adhesive material with plastic holding properties at the fusion bonding temperature, applying to said liquid adhesive film an excess of a mixture of a noneutectic metal alloy coating having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F. with a refractory material which remains solid at the fusion bonding temperature, said coating alloy and refractory material being granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh, removing from said article the excess of granulated mixture other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said coating alloy will displace said nonmetallic adhesive material and fusion bond with said metal article binding said refractory material in said coating.

5. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a volatile liquid vehicle, and a nonvolatile nonmetallic inorganic adhesive material with plastic holding properties at the fusion bonding temperature, applying to said liquid adhesive film an excess of refractory particles granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh having a melting point above said fusion bonding temperature, removing from said article the excess of said granulated refractory particles other than the substantially single layer retained by said adhesive film, applying to the exposed surface of said layer of refractory particles an additional film of said liquid adhesive, applying to said latter liquid adhesive film an excess of a noneutectic metal alloy coating granulated within a range of through 30 retained on 325 mesh having a substantial plastic temperature range between liquidus and solidus points below the melting pointof said metal article and within a temperature rangeof 1500 to 2500 F., removing from said article the excess of granulated coating alloy other than that retained by said second adhesive film, and

heating said surface to a temperature within said plastic range at which said coating alloy will displace said nonmetallic adhesive material and fusion bond with said metal article at least partially embedding said refractory particles in the resultant coating.

6. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a water solution of soluble proportions of water glass and borax in which is mixed to a paint consistency a metal powder having a particle size smaller than retained on 325 mesh and having solid characteristics at the fusion bonding temperature, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating granulated to a size exceeding the thickness of said adhesive film within a range of 325 to 30 mesh having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F., removing from said article the excess of granulated coating alloy other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said coating alloy will displace said nonmetallic adhesive mate rial and fusion bond with said metal article.

7. A process for fusion bonding an accurately controlled thickness of metal coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a volatile liquid vehicle, a non-volatile nonmetallic inorganic adhesive material with plastic holding properties at the fusion bonding temperature and a metal powder smaller than retained on 325 mesh having solid characteristics at the fusion bonding temperature mixed therein to a paint consistency, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating granulated in uniform particle size and within a range of through 30 retained on 325 mesh a single layer of which uniform size particles when fused into a continuous smooth layer will correspond to said desired thickness, said metal coating having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F., removing from said article the excess of granulated metal coating other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said metal coating will displace said nonmetallic adhesive material and fusion bond with said metal article smoothing out through surface tension to a continuous surface coating of said desired thickness.

8. A process for fusion bonding a particulate metal coating having a minimum exposed superficial area consistent with the final desired coating thickness onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a volatile liquid vehicle, a nonvolatile nonmetallic inorganic adhesive material with plastic holding properties at the fusion bonding temperature and a metal powder smaller than retained on 325 mesh having solid characteristics at the fusion bonding temperature mixed therein to a paint consistency, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh and of a uniform maximum particle size consistent with said perature range of 1500 to 2500' final desired coating thickness, said metal coating having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F., removing from said article the excess of granulated metal coating other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said metal coating will displace said nonmetallic adhesive material and fusion bond with said metal artlc e.

9. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a water solution of soluble proportions of water glass and borax in which is mixed to a paint consistency a metal powder having a particle size smaller than retained on 325 mesh and solid characteristics at the fusion bonding temperature, applying to said liquid adhesive film an excess of a mixture of noneutectic metal alloy coating having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F. with a refractory material which remains solid at the fusion bonding temperature, said metal coating and refractory material being granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh, removing from said article the excess of granulated mixture other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said metal coating will displace said nonmetallic adhesive material and fusion bond with said metal article binding said refractory material in said coating.

10. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a water solution of soluble proportions of water glass and borax in which is mixed to a paint consistency a metal powder having a particle size smaller than retained on 325 mesh and solid characteristics at the fusion bonding temperature, applying to said liquid adhesive film an excess of refractory particles granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh having a melting point above said fusion bonding temperature, removing from said article the excess of said granulated refractory particles other than the substantially single layer retained by said adhesive film, applying to the exposed surface of said layer refractory particles an additional film of said liquid adhesive, applying to said latter liquid adhesive film an excess ing granulated within a range of through 30 retained on 325 mesh having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a tem- F., removing from said article the excess of granulated metal coating other than that retained by said second adhesive film, and heating said surface to a temperature within said plastic range at which said metal coating will displace said nonmetallic adhesive material and fusion bond with said metal article at least partially embedding said refractory particles in the resultant coating.

11. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a volatile liquid vehicle, a nonvolatile nonmetallic'inorganic adhesive material with plastic holding properties at the fusion bonding temperature and a refractory metal powder less than retained on 325 mesh having solid characteristics at the fusion bonding temperature mixed therein to a paint consistency, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F., removing from said article the excess of granulated metal coating other than the of a noneutectic metal alloy coat-- substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said metal coating will displace said nonmetallic adhesive material and fusion bond with said metal article.

12. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a volatile liquid vehicle, a nonvolatile nonmetallic inorganic adhesive material with plastic holding properties at the fusion bonding temperature and a metal powder smaller than through 30 retained on 325 mesh having solid characteristics at the fusion bonding temperature mixed therein to a paint consistency, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F., removing from said article the excess of granulated metal coating other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said metal coating will displace said nonmetallic adhesive material and fusion bond with said metal article.

13. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a water solution of soluble proportions of water glass and borax, applying to said liquid adhesive film an excess of a noneutectic metal alloy coating granulated to a size exceeding the thickness of said adhesive film within a range of through 30 retained on 325 mesh having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500 to 2500 F., removing from said article the excess of granulated metal coating other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said metal coating will displace said nonmetallic adhesive material and fusion bond with said metal article.

14. A liquid adhesive specifically for use in a metal-tometal fusion bonding process carried out at temperatures in the order of 1500 to 2500 F. substantially as described consisting essentially of soluble proportions of commercial water glass and a saturated Water solution of borax diluted in the order of three parts by volume of water for each part of water glass and having a metal powder smaller than retained on 325 mesh with solid properties at the fusion bonding temperature mixed therein to substantially a paint consistency.

15. A liquid adhesive specifically for use in a metalto-metal fusion bonding process carried out at temperatures in the order of 1500 to 2500 F. substantially as described consisting essentially of soluble proportions of commercial water glass and borax in a water vehicle, the ratio of borax to waterglass not exceeding eight grams of borax per cc. of waterglass-40 Baum.

16. In a process for binding refractory particles having a relatively high melting point onto an exposed surface of a relatively high melting point base metal article with particles of a relatively low melting point metal binder fused onto said surface at least partially embedding said refractory particles, the method of heating to a fusion bonding temperature over 1500 F., consisting in submerging said article with the refractory particles and metal binder applied with a thin film of liquid adhesive to said surface in a fused boric oxide bath having said desired fusion bonding temperature.

17. In a process for fusion bonding -a metal coating of particles of relatively lower melting point onto an exposed surface of a relatively higher melting point metal article, the method of heating to a fusion bonding temperature over 1500 F., consisting 'in submerging said article with the metal coating applied with a thin film of liquid adhesive to said surface in a fused boric oxide bath having said desired fusion bonding temperature.

18. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated particles of a metal coating capable of fusing at a temperature over 1500 F. and below the melting point of said article, utilizing a water glass adhesive applied in liquid form and thereafter dried for holding a uniform coating of said particles during heating, and heating said surface to a temperature at which said metal coating will displace said adhesive and fusion bond with said metal article, said heating being effected by submerging said article in a fused bath of boric oxide having a temperature corresponding to said fusion bonding temperature.

19. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated particles of a metal coating capable of fusing at a temperature over 1500 F. and below the melting point of said metal article, utilizing essentially a sodium silicate adhesive including a water soluble proportion of borax applied in liquid form and therefore dried for temporarily retaining a uniform coating of said particles, and heating said surface to a temperature at which said metal coating will displace said adhesive and fusion bond with said metal article, said heating being effected by submerging said article in a fused bath of boric oxide having a temperature corresponding to said fusion bonding temperature.

20. In a process for fusion bonding a particulate metal coating onto the surface of a metal article wherein a liquid film of adhesive is employed to retain a uniform application of metal coating on said surface, the method of applying metal coating comminuted within the range of 20 to 44 microns consisting in first applying to the surface to be coated a uniform film of said liquid adhesive prepared with a paint consistency, and then gradually sprinkling said metal coating over said liquid film to an excess by screening on through a sieve,

and then removing the excess of said metal coating other I than the substantially uniform layer retained by said adhesive film drying said adhesive, and heating to the fusion bonding temperature.

21. A process for fusion bonding a coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated particles of a noneutectic metal alloy coating capable of fusing at a temperature over 1500 F. and having a substantial plastic range below the melting point of said metal article, utilizing essentially a liquid sodium silicate adhesive including a water soluble proportion of borax for temporarily retaining a uniform coating of said particles, drying said adhesive, and heating said surface to a temperature within said plastic range at which said metal coating will displace said adhesive and fusion bond with said metal article, said heating being effected by submerging said article in a fused bath of boric oxide having a temperature corresponding to said fusion bonding temperature.

22. A process for fusion bonding a particulate coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated at least one application of a film of liquid adhesive consisting essentially of water soluble proportions of water glass and borax into which a metal powder havmg a maximum particle size on the order of 20 microns and a melting point above the fusion bonding temperature has been mixed in sufficient quantity and with sufficient water to provide a paint consistency, applying to each fi1m of liquid adhesive an excess of coating materlal 1n granulated particles exceeding the thickness of sald adhesive film and within a size range of through 30 reta ned on 325 mesh, at least one application of said coating material including a least a substantial proportion of a noneutectic metal coating alloy having a substantial plastic temperature range between liquidus and sol1dus pomts below the melting point of said metal artlcle and withln a temperature range of 1500 to 2500" F., removing from said article after each application of granulated coating material the excess other than the substantially single layer retained by said adhesive fi1m, dry1n g said adhesive after such excess removal, and heating said surface to a temperature within said plastic range at which said no neutectic metal coating alloy will displace the nonvolat le water glass and borax ingredients and fus on bond with said metallic article by submerging said article in a fused bath of boric oxide having a corresponding temperature. 23. A process for fusion bonding a particulate coat ng onto a metal article which includes the steps of applying to any surface of said article desired to'be coated at least one application of a film of hquid adhesive consisting a noneutectic metal coating alloy having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article and within a temperature range of 1500to 2500" F., removing from said article after each application of granulated coating material the excess other than the substantially single layer retained by said adhesive film, and heating said surface to a temperature within said plastic range at which said noneutectic metal coating alloy will displace said nonmetallic adhesive material and fusion bond with said metal article.

2 E A process for fusion bonding a particulate coating onto a metal article which includes the steps of applying to any surface of said article desired to be coated, a film of liquid adhesive consisting essentially of a volatile liquid vehicle, a non-volatile, non-metallic inorganic adhesive material with plastic holding properties at the fusion bonding temperature, applying to said film of liquid adhesive an excess of coating material granulated to a size within the range of through 30 mesh to 20 microns, said coating material including at least a substantial proportion of a non-eutectic metal coating alloy having a 25. A process for fusion bonding a particulate coat- I ing onto a metal article which includes the steps of applying to any surface of said article desired to be coated a film of liquid adhesive consisting essentially of a volatile liquid vehicle, a non-volatile, non-metallic inorganic adhesive material with plastic holding properties at the fusion bonding temperature, applying to said film of liquid adhesive an excess of particulate coating material including at least a substantial proportion of non-eutec tic metal coating alloy having a substantial plastic temperature range between liquidus and solidus points below the melting point of said metal article, removing from said article the excess other than the substantially uniform layer retained by said adhesive film and heating said surface to a temperature within said plastic range at which said non-eutectic metal coating alloy will displace said non-metallic adhesive material and fusion bond with said metal article.

26. The process set forthin claim 25 wherein the liquid adhesive on said article is dried before heating and wherein a fused boric oxide heating bath is employed for heating said article to the fusion bonding temperature.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 110,727 Beck Jan. 3, 1871 242,649 Howes June 7, 1881 682,174 Coleman Sept. 10, 1901 1,735,000 Dely Nov. 12, 1929 1,817,888 Lowe Aug. 4, 1931 2,027,065 Sadtler Jan. 7, 1936 2,261,228 Cockrum Nov. 4, 1941 2,301,763 Wagner Nov. 10, 1942 2,323,169 Wagenhals June 29, 1943 2,339,208 Van Der Pyl Jan. 11, 1944 2,341,784 John Feb. 15, 1944 2,341,885 Sowa Feb. 15, 1944 2,378,252 Staehle et al June 12, 1945 2,440,969 Nightingall May 4, 1948 2,576,308 Nordon Nov. 27, 1951 FOREIGN PATENTS Number Country Date 19,597 Great Britain of 1900 474,064 Great Britain Jan. 17, 1936 104,231 Australia June 30, 1938 

1. A PROCESS FOR FUSION BONDING A METAL COATING ONTO A METAL ARTICLE HAVING A RELATIVELY HIGHER MELTING POINT WHICH INCLUDES THE STEPS OF APPLYING TO ANY SURFACE OF SAID ARTICLE DESIRED TO BE COATED A FILM OF LIQUID ADHESIVE CONSISTING ESSENTIALLY OF WATER SOLUBLE PROPORTION OF WATER GLASS AND BORAX INTO WHICH A METAL POWDER HAVING A MAXIMUM PARTICLE SIZE IN THE ORDER OF 20 MICRONS AND A MELTING POINT ABOVE THE FUSION BONDING TEMPERATURE HAS BEEN MIXED IN SUFFICIENT QUANTITY AND WITH SUFFICIENT WATER TO PROVIDE A PAINT, CONSISTENCY, APPLYING TO SAID LIQUID ADHESIVE FILM AN EXCESS OF A NONEUTECTIC METAL ALLOY COATING IN GRANULATED PARTICLES EXCEEDING THE THICKNESS OF SAID ADHESIVE FILM AND WITHIN A RANGE OF THROUGH 30 RETAINED ON 325 MESH HAVING A SUBSTANTIAL PLASTIC TEMPERATURE RANGE BETWEEN LIQUIDUS AND SOLIDUS POINTS BELOW THE MELTING POINT OF SAID METAL ARTICLE AND WITHIN A TEMPERATURE RANGE OF 1500* TO 2500* F., REMOVING FROM SAID ARTICLE THE EXCESS OF GRANULATED COATING ALLOY OTHER THAN THE SUBSTANTIALLY SINGLE LAYER RETAINED BY SAID ADHESIVE FILM, DRYING SAID ADHESIVE, AND HEATING SAID SURFACE TO A TEMPERATURE WITHIN SAID PLASTIC RANGE AT WHICH SAID COATING ALLOY WILL DISPLACE THE NONVOLATIEL WATER GLASS AND BORAX INGREDIENTS AND FUSION BOND WITH SAID METAL ARTICLE BY SUBMERGINE SAID ARTICLE IN A FUSED BATH OF BORIC OXIDE HAVING A CORRESPONDING TEMPERATURE. 